The present invention is directed to optical devices and related methods. More particularly, the present invention provides a method and device for emitting electromagnetic radiation for optical devices using non-polar or semipolar gallium containing substrates such as GaN, AlN, InN, InGaN, AlGaN, and AlInGaN, and others. Merely by way of example, the invention can be applied to optical devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices.
In the late 1800's, Thomas Edison invented the lightbulb. The conventional lightbulb, commonly called the “Edison bulb,” has been used for over one hundred years. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to an AC power source or a DC power source. The conventional light bulb can be found commonly in houses, buildings, and outdoor lighting applications, and other areas requiring light. Unfortunately, drawbacks exist with the conventional Edison light bulb. That is, the conventional light bulb dissipates much thermal energy leading to inefficiencies. More than 90% of the energy used for the conventional light bulb dissipates as thermal energy. Additionally, the conventional light bulb routinely fails often due to thermal expansion and contraction of the filament element.
To overcome some of the drawbacks of the conventional light bulb, fluorescent lighting has been developed. Fluorescent lighting uses an optically clear tube structure filled with a halogen gas. A pair of electrodes is coupled between the halogen gas and couples to an alternating power source through a ballast. Once the gas has been excited, it discharges to emit light. Often times, the optically clear tube is coated with phosphor materials. Many building structures use fluorescent lighting and, more recently, fluorescent lighting has been fitted onto a base structure, which couples into a standard socket.
Solid state lighting techniques have also been used. Solid state lighting relies upon semiconductor materials to produce light emitting diodes, commonly called LEDs. At first, red LEDs were demonstrated and introduced into commerce. Red LEDs use Aluminum Indium Gallium Phosphide (or AlInGaP) semiconductor materials. Most recently, Shuji Nakamura pioneered the use of InGaN materials to produce LEDs emitting light in the blue color range for blue LEDs. The blue colored LEDs lead to innovations such as the BlueRay™ DVD player, solid state white lighting, and other developments. Other colored LEDs have also been proposed.
High intensity green LEDs based on GaN have been proposed and even demonstrated with limited success. Unfortunately, achieving high intensity, high-efficiency GaN-based green LEDs has been problematic. The performance of optoelectronic devices fabricated on conventional c-plane GaN suffer from strong internal polarization fields, which leads to poor radiative recombination efficiency. Since this phenomenon becomes more pronounced in InGaN layers with increased indium content for increased wavelength emission, extending the performance of GaN-based LEDs to the green regime has been difficult. Furthermore, increased indium content in a GaN film often requires reduced growth temperature leading to poorer crystal quality of high-indium-content InGaN films. The difficulty of achieving a high intensity green LED has lead scientists and engineers to the term “green gap” to describe the generally unavailability of such green LED.